Applied Wetlands Science - Chapter 9 doc

Constructed wetlands have been successfully used as treatment systems fordomestic wastewater effluent, from single-residence wetlands to large municipalwastewater treatment facilities.. A

DeBusk, Thomas A et al “Wetlands for Water Treatment”

Applied Wetlands Science and Technology

Editor Donald M Kent

Boca Raton: CRC Press LLC,2001

CHAPTER 9 Wetlands for Water TreatmentThomas A DeBusk and William F DeBusk

Biological Removal ProcessesChemical Removal ProcessesPlanning and Design

Treatment Wetlands as a Unit ProcessRegulatory Issues

Preliminary Feasibility and Alternatives AnalysesDesign Considerations

Construction and ManagementConstruction

ManagementPerformanceSuspended Solids and Organic Carbon RemovalRemoval of Organic Carbon and Suspended Solids from Waste

Stabilization Pond Effluents

Wetlands are widely regarded as biological filters, providing protection for waterresources such as streams, lakes, estuaries, and groundwater Although naturallyoccurring wetlands have always served as ecological buffers, research and develop-ment of wetland treatment technology is a relatively recent phenomenon Studies ofthe feasibility of using wetlands for wastewater treatment were initiated during theearly 1950s in Germany In the United States, wastewater to wetlands research began

in the late 1960s and increased dramatically in scope during the 1970s As a result,the use of wetlands for water and wastewater treatment has gained considerablepopularity worldwide Currently, an estimated 1000 wetland treatment systems, bothnatural and constructed, are in use in North America (Cole, 1998)

The goal of water and wastewater treatment is the removal of aqueous inants in order to decrease the possibility of detrimental impacts on humans and therest of the ecosystem The term contaminant is used in this context to refer to anyconstituent in the water or wastewater that may adversely affect human and envi-ronmental health Many contaminants, including a wide variety of organic com-pounds and metals, are toxic to humans and other organisms Other types of con-taminants may not be hazardous in the conventional sense but nevertheless pose anindirect threat to our well being For example, loading of nutrients (e.g., nitrogenand phosphorus) to waterways can result in excessive growth of algae and unwantedvegetation This growth diminishes the recreational, economic, and aesthetic values

contam-of lakes, bays, and streams

Constructed wetlands have been successfully used as treatment systems fordomestic wastewater effluent, from single-residence wetlands to large municipalwastewater treatment facilities Similarly, wetlands may be used effectively fortreatment of animal and aquaculture wastes The use of wetland retention basins fortreatment of stormwater runoff has become relatively commonplace The composi-tion of stormwater varies greatly, depending on the surrounding land use For exam-ple, urban runoff may contain soil particles, dissolved nutrients, heavy metals, oil,and grease Residential and agricultural runoff may also contain organic matter andpesticides A variety of industrial wastes, including pulp and paper, food processing,slaughtering and rendering, chemical manufacturing, petroleum refining, and landfillleachates are amenable to wetland treatment

While wetlands are remarkable in their ability to treat diverse types of inated waters, there are limitations that should be carefully considered prior to theimplementation of any wetland treatment system Pretreatment, for example, primarysedimentation or anaerobic or aerobic stabilization, is often required prior to feedingdomestic and industrial effluents into a wetland Selected water contaminantsremoved within a treatment wetland ultimately may become available for assimila-tion and potentially be toxic to wetland biota A sound understanding of wetlandcontaminant removal processes (Reddy and D’Angelo, 1997), the long-term fate ofthese contaminants, and contaminant removal effectiveness of various wetland types

contam-is critical in the proper design and operation of treatment wetlands

GENERAL FEATURES OF WETLANDS THAT CONTRIBUTE TO

CONTAMINANT REMOVAL

The unique combination of structural and functional attributes sets wetlandsapart from terrestrial and aquatic ecosystems in their ability to remove or sequesternutrients and toxic environmental contaminants For example, shallow water, lowcurrent velocity, and the physical filtering action of plant stems and leaves providefavorable conditions for settling of particulate matter Wetlands also provide sub-strates for a multitude of chemical and microbiological processes, promoting nutrientremoval and storage within the complex maze of microsites in the soil and vegetationcover The total surface area available for microbial activity in the soil and theoverlying dead plant material (litter or detritus) is extremely high in wetlands.Physical, chemical, and microbiological processes are further enhanced in wetlands

by retention of water for extended periods within this biologically active zone.Another important characteristic of wetlands is the presence of anaerobic (oxygen-depleted) soils during periods of flooding which gives rise to an aerobic–anaerobicinterface, or boundary, near the soil surface This juxtaposition of aerobic andanaerobic conditions provides an environment for unique chemical and microbio-logical reactions that greatly enhance the removal of nutrients from inflowing water.Wastewater-borne labile organic carbon compounds, expressed as biochemicaloxygen demand (BOD), are readily removed in anaerobic and aerobic microenvi-ronments of treatment wetlands Reduced nitrogen (N) compounds (e.g., ammonium)are nitrified in aerobic regions, from which the products can migrate (either by bulktransfer or diffusion) to anaerobic regions Denitrification of the produced NOx

species occurs rapidly due to the preponderance of anaerobic conditions and readyavailability of labile carbon compounds from decaying vegetation and organic soils

In this sequential N removal process, nitrification typically is the rate-limiting stepdue to low oxygen availability in many parts of the system

Plant uptake and adsorption to soil surfaces contribute to short-term phosphorus(P) removal in wetlands However, the only prominent, long-term P sink is thought

to occur through soil accumulation Large wetland areas are therefore required toachieve substantial P removal

Treatment wetlands differ in two fundamental ways from more conventionalwastewater treatment unit processes First, wetlands sacrifice consistently high

microorganism densities and strict process control for reduced construction costsand operator attention Second, solids processing occurs internally in wetlands, so

no biological sludge management is required, at least on the short-term For theabove reasons, treatment wetlands have moderate to high land requirements

In summary, a number of physical, chemical, and biological processes operateconcurrently in constructed and natural wetlands to provide contaminant removal(Figure 1) Removal of contaminants may be accomplished through storage in thewetland soil and vegetation or through losses to the atmosphere Knowledge of thebasic contaminant removal concepts is extremely helpful for assessing the potentialapplications, benefits, and limitations of wetland treatment systems These processesare described in more detail in a later section

TYPES OF TREATMENT WETLANDS

Treatment wetlands are generally classified as either free water surface (FWS) orsubsurface flow (SSF) systems (Figure 2) Subsurface flow wetlands are the commonsystem design implemented in Europe for domestic wastewater treatment which hasgreater than 500 treatment wetlands In North America, with around 600 treatmentwetlands, the FWS type is more common (Cole, 1998) In the United States, FWSwetlands for domestic wastewater treatment commonly occur in communities with

1000 or fewer people, although some large FWS wetlands exist in cities with lations greater than 1 million As of late 1998, South Dakota was the state with thegreatest number of operational (nonpilot) FWS wetlands (42), followed by Florida(24) and California (11) (U.S Environmental Protection Agency, 1999)

popu-The widespread use of treatment wetlands in South Dakota, a state with harshwinter conditions, provides a good indication of the versatility of treatment wetlands,

as well as the circumstances under which they are an appropriate and competitivetechnology Because of low capital (owing to inexpensive land) and operating costsand the ability to provide winter water storage, waste stabilization ponds (WSPs)have been widely implemented for domestic wastewater treatment during the pastfour decades As of 1991, 246 communities in South Dakota were using WSPs Adrawback of WSPs is that they do not consistently provide low effluent suspendedsolids, ammonium, and total phosphorus concentrations A strong interest in addi-tional protection of the quality of water resources, as well as in creating new wildlifehabitat, has led to the upgrading of many WSPs with treatment wetlands during thepast decade (Dornbush, 1993) The South Dakota state regulatory agency has encour-aged the use of constructed wetlands by providing design guidelines and economicassistance with treatment wetland construction

Free Water Surface Wetlands

FWS design typically incorporates a shallow layer of surface water, flowing overmineral (sandy) or organic (peat) soils Vegetation often consists of marsh plants,such as Typha (cattails) and Scirpus (bulrush), but may also include floating andsubmerged aquatic vegetation and wetland shrubs and trees (Figure 3) Natural

- P (with Fe, Al, Ca)

- Metals (with sulfides)

SURFACE WATER DETRITUS (LITTER)

SOIL

wetlands, both forested and herbaceous, have also been effectively used as FWStreatment wetlands.

For some treatment applications, FWS wetlands are designed and managed toencourage dominance by either floating or submerged macrophytes Water depth isone parameter that can be controlled to discourage emergent macrophytes, therebyallowing development of either a floating aquatic macrophyte (FAM) or submergedaquatic vegetation (SAV) system

Free water surface wetlands vary dramatically in size, from less than 1 ha togreater than 1000 ha Large FWS wetlands are even being used as a nutrient controltechnology to treat runoff from entire regional watersheds For example, over 16,000

ha of FWS wetlands are being constructed in South Florida to remove P fromagricultural drainage water before it enters the Everglades (Moustafa et al., 1999).Free water surface wetlands offer ecological and engineering benefits beyondwater treatment Free water surface wetlands used for treating agricultural and urban

Figure 2 Schematic of free water surface (FWS) and subsurface flow (SSF) wetlands.

FREE WATER SURFACE WETLAND

OR PEAT)

WATER LEVEL

DETRITUS (LITTER) GRAVEL

OR SOIL

runoff also reduce hydraulic runoff peaks from storm events Effectiveness depends

on the wetland size (volume), location in the watershed, and configuration of inletand outlet structures (Schueler, 1996) Many FWS treatment wetlands provide both

a recreational amenity and wildlife habitat Iron Bridge wetland in Orlando, FL andthe Arcata marsh in Humbolt, CA have each provided water treatment and otherecological and aesthetic benefits for more than a decade (Jackson, 1989; Gearhartand Higley, 1993)

Subsurface Flow Wetlands

Subsurface flow wetlands differ from FWS wetlands in that they incorporate arock or gravel matrix that the wastewater is passed through in a horizontal or verticalfashion (see Figure 2) Unless the matrix clogs, the top layer of the bed in horizontalflow systems will remain dry The SSF configuration offers several advantages,including a decreased likelihood of odor production and no insect proliferation withinthe wetland as long as surface ponding is avoided Unlike FWS wetlands, SSFsystems provide no aesthetic or recreational benefits and few, if any, benefits towildlife

Subsurface flow wetlands continue to provide effective treatment of most water constituents through the winter in temperate climates The subsurface micro-bial treatment processes still function, albeit at a reduced rate, even when the surfacevegetation has senesced or died, and the matrix surface is covered with snow andice Subsurface flow wetlands also can be operated in a vertical flow fashion which

waste-Figure 3 Free water surface (FWS) wetlands incorporate a shallow layer of surface water

and vegetation such as these emergent macrophytes.

can reduce matrix clogging problems and enhance certain contaminant removalprocesses such as nitrification.

Because of the high cost of the gravel or rock matrix, SSF wetlands never attainthe large spatial footprint of the large FWS wetlands Concerns over matrix cloggingand the potential high cost of renovation also limit the deployment of extremelylarge SSF wetlands However, SSF are finding increased use for small applications,such as for small communities or single family homes The limitations of septicsystems for nutrient control have become more apparent in the past two decades(Hagedorn et al., 1981), and SSF wetlands are one technology that is being deployed

to improve nutrient removal performance (Mitchell et al., 1990) Subsurface flowsystems are the only wetland configuration suitable for this purpose, because theycreate no standing water, thereby limiting the likelihood of human exposure towastewater pathogens (House et al., 1999)

Hybrid Treatment Wetlands

A number of treatment wetlands have been constructed that combine differentwetland types Some, such as the 134-ha Eastern Service Area wetland in Orlando,

FL, consist of constructed FWS wetlands that are followed by natural forestedwetlands This particular configuration was based on regulatory needs, with thenatural parcel receiving water only after pretreatment by the constructed wetland(Schwartz et al., 1994) Other hybrid systems are based on specific contaminantremoval needs For example, the key to enhanced nitrogen removal in SSF wetlands

is to create an intermediate step in the process train with an oxygenated environmentthat enhances nitrification in the rock or gravel matrix Many subsurface wetlandsaccomplish this by sequencing horizontal flow beds with vertical flow beds or byrecirculating partially treated effluent onto an inflow region rock or sand filter toenhance nitrification (Cooper et al., 1997; Reed and Brown, 1995) Other investiga-tors have recommended operating vertical flow beds with various draw and fill cycles

to enhance chemical oxygen demand (COD) and nitrogen removal, but performanceusing this technique has been mixed (Burgoon et al., 1995; Boutin et al., 1997).Regardless of the approach used to stimulate nitrification, the SSF wetland is con-figured so that the nitrate-rich effluent subsequently is introduced into an anoxicsection of the bed where denitrification readily occurs

TREATMENT WETLAND COMPONENTS

Treatment Wetland Vegetation

Macroscopic vegetation is the most prominent feature of treatment wetlands.Free water surface wetlands can develop as simple monocultures of weedy orcompetitive species such as Typha (cattail) or Phragmites (reed), but more oftenthey contain a diversity of other emergent and floating plants within genera such

as Pontederia, Sagittaria, Eleocharis, Utricularia, and Lemna Treatment wetlandstypically are planted just prior to initial flooding to ensure rapid vegetative cover

development and to facilitate initiation of water treatment Under extreme operatingconditions, such as high organic, nutrient, or hydraulic loading rates, the systemmay remain a monoculture or near monoculture for the life of the treatment system.Similarly, subsurface flow wetlands usually remain dominated by the speciesplanted prior to startup, typically Phragmites (reed), Scirpus (bulrush), or Typha

(cattail) This is due both to the difficulty of seeds and other propagules in becomingestablished on the bed’s surface and the often high organic loading provided to SSFsystems

Under less rigorous environmental conditions, the vegetative community thatdevelops over time in FWS wetlands may bear little resemblance to the speciesoriginally planted At the Eastern Service Area Treatment Wetland (Orlando, FL),one constructed wetland is quite shallow and experiences periodic drydown, and thesecond, while also shallow, is continuously inundated This system is used for furtherpolishing of domestic wastewater that has received conventional, advanced treatment

to levels below 5 mg BOD/l, 5 mg total suspended solids (TSS)/l, 3 mg N/l, and

1 mg P/l (Schwartz et al., 1994) Upon wetland startup, 13 species were planted intothe mineral soils at a density of 336 plants per ha The vegetative communities inthe two constructed wetlands were sampled at 1 and 4 years after planting Onlyone of the most abundant species occurring at year four in each wetland was aspecies that was originally planted (Table 1)

In a wetland ecosystem self-organization experiment, Mitsch et al (1998) lished two individual 1-ha wetlands for treating Olentangy River water in Ohio Onewetland was planted with 2400 propagules (rootstock and rhizomes), representing

estab-13 plant species at an overall density of 0.24 plants/m2 Species planted included

Nelumbo lutea, Nymphaea odorata, and Potamogeton pectinatus in the deepest(0.6 m depth) region, Scirpus validus and Scirpus fluviatilis at moderate (0.3 m)depth, and Spartina pectinata, Sparganium eurycarpum, Acorus calamus, Sagittaria latifolia, Pontedereria cordata, Juncus effusus, Saururus cernuus, and Cephalanthus occidentalis in the shallow, littoral (0 to 0.3 m) region The second wetland, adjacent

to the first, was left unplanted The wetlands were evaluated each year after

start-up for water treatment aspects and flora and fauna characteristics As of year three,

9 of the 13 original stocked species in the planted wetland were still present, althoughthe total number of macrophyte species had increased to 65 (Mitsch et al., 1998).The unplanted wetland had similar vegetation at year three, and had 54 macrophytespecies However, only 1 of the 13 species originally planted in the adjacent wetlandwere present Treatment performance and fauna of the two systems were remarkablysimilar at year three

Whether or not to plant a FWS treatment wetland upon start-up, as well as whatdensity to plant, is dictated by the urgency to achieve an operational system If thesystem is built a year or two in advance of water or wastewater treatment needs,and the design does not call for any specific vegetation components, then existingstudies clearly show that natural recruitment can be relied upon for development of

a diverse plant community Depending on the depth and nutrient regime of thewetland, mats of filamentous algae, phytoplankton, or submerged macrophytes likelywill dominate in the wetland water column prior to development of a dense emergentmacrophyte community

From a water treatment perspective, macrophyte vegetation provides a number

of functions (Brix, 1997) In FWS wetlands, dense macrophyte stands shade thewater column reducing or eliminating phytoplankton populations Conversely, insparse macrophyte stands, the emergent stems may serve as attachment sites forperiphytic algae In either case, the submerged plant portions also provide surfacearea for colonization by bacteria that contribute to processing of carbon, nitrogen,and other wastewater constituents

Emergent and floating macrophytes shield the water from direct sunlight and,therefore, moderate the temperature of the shallow water column These plants alsotend to dissipate or block wind and wave energy and, therefore, help maintainquiescent conditions in the water column This promotes settling of wastewater-borne solids and inhibits resuspension of flocculant sediments from the bottom

Table 1 Characteristics of Herbaceous Vegetation Communities in

Two Shallow Eastern Service Area Wetlands, Orlando, FL

Frequency Rank P/R Year 1 Year 4

Values represent frequency of occurrence rankings in May 1988

and December 1992, 1 and 4 years after planting The first column

denotes whether the species was originally planted (P) or naturally

recruited (R).

* NF means not found.

Source: Wallace, Ecosystem Research Corporation, Gainesville, FL.

Some floating species, such as the duckweeds, provide a dense floating mat on thewater’s surface that can inhibit oviposition by mosquitoes and even act as a barrier

to prevent escape of odors produced in the bottom sediments or water column.The contribution of belowground plant tissues, rhizomes, and roots to wastewatertreatment depends on the system configuration In FWS wetlands, the roots ofemergent plants assimilate nutrients from the soil porewater and also provide ananchoring function that can reduce erosion In a SSF wetland or a FWS wetlanddominated by floating plants, the roots are in intimate contact with the wastewaterand O2 leakage from the roots can enhance microbial processes (e.g., nitrification)that require oxic conditions Oxygen is transported internally to the plant roots either

by passive molecular diffusion or convective (bulk) flow of air through the internallacunae The convective air flow can be driven by a number of physical processes,including wind velocity gradients in the plant canopy and temperature or humiditydifferences between the interior and exterior of the plant (Brix, 1993, 1994)

In SSF and FAMs systems, the plant roots are in intimate contact with thewastewater In these systems, the diffuse root mats harbor bacteria which may alsobenefit from oxygen transported from the foliage to the rhizosphere For thesesystems, a number of investigations have been conducted to determine the effective-ness of various vegetation types on treatment performance Gersberg (1985) con-ducted one of the first studies to compare effectiveness of cattail (Typha) and bulrush(Scirpus) for ammonia removal in a SSF treatment wetland Using large gravel bedsystems receiving domestic wastewater, these investigators found that bulrush (Scir- pus) beds provided greater ammonia removal than either unvegetated beds or bedscontaining cattail (Typha) A nitrogen mass balance revealed that macrophyte uptakeaccounted for only a small percentage of the N removed from the wastewater, sothe higher nitrification rate for the Scirpus bed was attributed to this specie’s greateroxygen transport capacity

Results from numerous small-scale studies have demonstrated that the species

of plants used in treatment wetlands can affect system contaminant removal mance, particularly for the nutrients N and P Both emergent and floating macro-phytes have been rigorously characterized with respect to their N and P uptakecapability For most nutrient-laden waters in moderate climates, water hyacinth(Eichhornia crassipes) and water lettuce (Pistia stratiotes) provide the highest rates

perfor-of N and P uptake among floating species (DeBusk et al., 1996a; Reddy and DeBusk,1985) Among emergent macrophytes, cattail (Typha), bulrush (Scirpus), and reed(Phragmites) provide some of the highest N and P removal rates Short-term P uptakerates in excess of 37 g P/m2 per year have been reported for these productive floatingand emergent species (Reddy and DeBusk, 1985; Tanner, 1996)

Despite obvious between-species differences in nutrient uptake, assessments offull-scale treatment wetlands reveal few differences in contaminant removal perfor-mance between wetlands dominated by different plant species This is because inthe long-term, most of the nutrients assimilated by the plant standing crop arerecycled by plant senescence, detritus production, and decomposition back into thewater column and sediment compartments What does seem to affect contaminantremoval performance on a large scale is overall plant habit, that is, whether emergent,submerged, or floating species dominates the plant community These differences

in habit influence water column and sediment-water interface environmental tions, such as dissolved oxygen concentrations and solar radiation inputs, that arethe critical master factors in controlling element cycling and contaminant removal

condi-in wetlands (Reddy and D’Angelo, 1997; Reddy et al., 1999)

Finally, where prominent plant morphological differences exist (or differences

in survival) in a given waste stream, performance differences for a particular planthabit (e.g., floating or emergent) will occur For example, floating aquatic macro-phyte systems dominated by large-leafed water hyacinth and pennywort providesuperior BOD removal performance compared to small-leafed duckweeds This islikely because of the greater surface area of underwater rhizomes and roots andgreater foliage to rhizosphere O2 transport by the larger-leafed floating species(Clough et al., 1987)

Hydroperiod and Hydraulics

In the natural landscape, hydroperiod (frequency and duration of flooding) is aprominent factor that dictates wetland occurrence and characteristics Factors thatinfluence natural wetland hydroperiod include surface water and groundwater inputsand losses Additionally, the total annual volume of rainfall and evapotranspiration,

as well as the seasonal distribution of these atmospheric water fluxes, will influencethe type of wetland that occurs

Hydroperiod characteristics are less of a concern for wetlands that receive arelatively constant hydraulic loading, such as from a domestic wastewater source.Such wetlands often are isolated from groundwaters Moreover, evapotranspirationand rainfall generally need to be accounted for only in assessing their influence onthe effluent quality and mass contaminant removal budget and to ensure storage forlarge rainfall events (e.g., 25- or 100-year storm events) Atmospheric water fluxesalso must be carefully understood and addressed where unusual climatic (extremelylow annual rainfall) or site-specific conditions (highly permeable soils) exist.Treatment wetland hydraulics relate to the ability of the wetland to physicallyaccommodate water inputs, as well as internal reactor design characteristics thatcontribute to contaminant removal Because FWS wetlands are shallow basins,typically 0.5 to 1.5 m deep with the water column occupied in part by macrophytes,water passing through the wetland is subject to a certain amount of friction-inducedheadloss Shallow areas and areas with dense vegetation provide the most resistance

to flow Headloss in FWS wetlands will not be a major design concern unless thehydraulic loading rate is unusually high, the wetland aspect ratio (length to widthratio) is high, or the flow path extremely long By contrast, careful hydraulic design

is paramount in SSF wetlands where all of the flow is being routed through a gravel

or rock matrix Common parameters related to the hydraulic design of treatmentwetlands include hydraulic loading rate (HLR, usually expressed in cm/day) andhydraulic retention time (HRT, units usually in days) The former is obtained bydividing the flow (Q) by the wetland area (A); the latter is calculated by dividingthe flow by the water volume (V) of the wetland

Performance forecast modeling of treatment wetlands is based on the conceptthat these systems behave as plug-flow reactors, with flow moving in lock step

through the treatment wetland However, tracer studies conducted with emergent,floating, and submerged macrophyte-dominated treatment wetlands (DeBusk et al.,1990; Kadlec and Knight, 1996) reveal that flow patterns depart widely from idealplug-flow characteristics Temperature-related water column density gradients, theheterogeneous and clumped nature of vegetation, and uneven microtopographicalfeatures result in the development of rapid flow paths and internal dispersion andmixing (Kadlec, 1990) The net outcome is that some of the influent water reachesthe effluent end of the system long before the calculated hydraulic retention time(HRT), and a considerable amount is held longer than the calculated HRT From aperformance-forecasting standpoint, these deviations from plug-flow have beenaddressed by using different hydraulic reactor models For example, HRT has beenmodeled using several continuously stirred tank reactors (CSTRs) in series, or aplug-flow reactor followed by multiple CSTRs (Kadlec, 1997; King et al., 1997).Recognition of nonideal flow characteristics, as documented by full-scale tracerstudies, has led to most treatment wetlands being designed with a means of evenlydistributing the influent across the entire width of the wetland Once water entersthe wetland, however, flows coalesce into small rills that then combine to createlarge short-circuiting channels These flow channels typically remain intact until thewater is redistributed by structural means Both deep channels and earthen bermsperpendicular to flow have been used to redistribute water in wetlands (Kadlec andKnight, 1996) However, neither rational design parameters nor performance benefitsfor these structural modifications have been rigorously characterized Nevertheless,large treatment wetlands are often compartmentalized for purposes of improvingflow distribution, as well as to facilitate dry-down and maintenance of selectedportions of the treatment system (Figure 4).

Treatment Wetland Soils

Surface flow treatment wetlands can be constructed on both mineral and organicsoils Organic soils are generally categorized as having greater than 12 to 20 percentorganic matter content, a pH less than 6.0, low bulk density, and high water holdingand cation exchange capacities (Faulkner and Richardson, 1989) Mineral soils have

a low organic matter content, high bulk density, and often provide greater nutrientavailability than organic soils

Soils in FWS treatment wetlands serve several functions Soils must provideappropriate physical and chemical support for the emergent macrophytes The soilshould have physical properties that facilitate planting and recruitment of the aquaticvegetation and that physically can support the plants under flooded conditions Thesoil also must provide adequate nutrition to support macrophyte growth, particularlywhen treating waters of unusual chemical composition (e.g., industrial wastewaters)that may be deficient in certain plant macro- and/or micronutrients

Treatment wetland subsoils must exhibit low permeability In most instances,unless treating exceptionally clean waters or effluents, the treatment wetland should

be isolated from groundwaters (aquifers) Therefore, the soil profile should contain

an impermeable layer (hydraulic conductivity of less than 10–6 cm/s) that inhibitsvertical water movement If the soil profile is overly permeable and the internal

wastewaters must be isolated from groundwaters, then a clay, bentonite, or vinylliner can be incorporated into the wetland Permeability of the site soils will alsodictate their suitability for use in constructing berms.

Because soil is a prominent storage component (for nutrients etc.) in a FWSwetland, a soil type must be selected that does not add undesirable contaminants tothe overlying water column Extreme examples of soil nutrient export have actuallyresulted in treatment wetland use being curtailed Operation of a forested wetland

in central Florida that received advanced secondary domestic effluent was halteddue to an export of P and organic N This hydrologically altered wetland had beendry for more than 10 years prior to rehydration with wastewater effluent Waterquality transects within the wetland and laboratory soil column incubations revealedthat the exported compounds resulted from flooding the highly oxidized soils Almost

1 year of water exchange was required to reduce export of soil constituents to thewater column (Figure 5)

Some of the large wetlands in South Florida constructed on muck soils, ously used for agricultural crops, have also exhibited an increase in water columnnutrient levels upon flooding The Everglades Nutrient Removal Project, a 1370-hawetland designed to reduce P in agricultural drainage waters from the range of 150

previ-to 200 µg/l to 50 µg/l, exhibited water column P levels up to 370 µg/l in the first

2 months after flooding (Koch, 1991) Within 10 months, water column P trations had declined to 46 µg/l Experience with these wetlands demonstrates thathistorical land use and soil management practices (e.g., fertilization of agriculturalfields) can influence wetland water quality during the start-up phase

concen-Figure 4 The Iron Bridge wastewater polishing wetland in Orlando, FL has compartments

to improve flow distribution.

Regardless of the system design approach and type of vegetation that is aged, large FWS wetlands usually provide some habitat for fauna Previous land usepractices, therefore, should be assessed not only based on nutrient control, but alsofrom a wildlife health aspect For example, residual pesticides in previously farmedvegetable farms soils may be released into the surface water upon flooding, creating

encour-a hencour-azencour-ard to wencour-aterfowl encour-and other wildlife using the wetlencour-and

TREATMENT WETLAND CONTAMINANT REMOVAL PROCESSES

An understanding of wetland contaminant removal processes can facilitate land design, and aid dramatically in system troubleshooting should contaminantremoval performance not be as expected Wetlands provide effective transformationand storage of many water-borne constituents Contaminants removed from theinflow are either re-exported in an aqueous, but more innocuous form (e.g., chlorine

wet-to chloride), are swet-tored in the sediments (e.g., P, metals), or are lost from the system

in a gaseous form (e.g., methane, carbon dioxide, nitrogen gas) A number ofphysical, biological, and chemical processes are responsible for contaminant removal

in wetlands

Physical Removal Processes

Wetlands are capable of providing highly efficient physical removal of inants associated with particulate matter in the water or waste stream Surface watertypically moves very slowly through wetlands due to the characteristic broad sheetflow and the resistance provided by rooted and floating plants Sedimentation of

contam-Figure 5 Profiles of total organic carbon (TOC) with distance through a forested treatment

wetland 3 and 6 months after flooding with advanced secondary domestic water effluent Prior to rehydration, the wetland had been hydrologically altered (dry) for more than 10 years.

waste-suspended solids is promoted by the low flow velocity and by the fact that the flow

is often laminar (not turbulent) in wetlands Mats of floating plants in wetlands mayserve, to a limited extent, as sediment traps, but their primary role in suspendedsolids removal is to limit resuspension of settled particulate matter

Efficiency of suspended solids removal is proportional to the particle settlingvelocity and the length of the wetland For practical purposes, sedimentation isusually considered an irreversible process, resulting in accumulation of solids andassociated contaminants on the wetland soil surface However, resuspension ofsediment may result in the export of suspended solids and yield a somewhat lowerremoval efficiency Some resuspension may occur during periods of high flowvelocity in the wetland More commonly, resuspension results from wind-driventurbulence, bioturbation (disturbance by animals and humans), and gas lift Gas liftresults from production of gases such as oxygen from photosynthesis, and methaneand carbon dioxide produced by microorganisms in the sediment during decompo-sition of organic matter For some wetlands, build-up of sediment to detrimentallevels can occur, necessitating dry-down and sediment consolidation

Biological Removal Processes

Biological removal processes represent a prominent pathway of contaminantremoval in wetlands Probably the most widely recognized biological process forcontaminant removal in wetlands is plant uptake Wetland plants readily take upcontaminants that are also essential nutrients, such as nitrate, ammonium, and phos-phate However, certain wetland plant species are also capable of uptake and evensignificant accumulation of certain toxic metals such as cadmium and lead The rate

of contaminant removal by plants varies widely, depending on the plant growth rateand the concentration of the contaminant in the plant tissue Woody plants, that is,trees and shrubs, provide relatively long-term storage of contaminants comparedwith herbaceous plants However, contaminant uptake rate per unit area of land isoften much higher for herbaceous macrophytes such as Typha Algae may alsoprovide a significant amount of nutrient uptake but are relatively susceptible to thetoxic effects of heavy metals Storage of nutrients in algae is relatively short-term,due to the rapid turnover (life cycle) of the algae Bacteria and other microorganismsalso provide uptake and short-term storage of nutrients and some other contaminants

in the soil

As plants age and eventually die, dead plant material, known as detritus or litter,accumulates at the soil surface Some of the nutrients, metals, or other elementspreviously removed from the water by plant uptake are lost from the plant detritusand recycled back into the water Leaching of water-soluble contaminants may occurrapidly upon the death of the plant or plant tissue, while a more gradual loss ofcontaminants occurs during decomposition of detritus by bacteria and other organ-isms Recycled contaminants may be flushed from the wetland in the surface water

or may be removed again from the water by biological uptake or other means

In most wetlands, there is a net accumulation of plant detritus because the rate

of decomposition is substantially reduced relative to upland ecosystems by the lowavailability of oxygen for the decomposers Anoxic and anaerobic conditions

generally prevail in wetland soils because the extremely low diffusion rate of oxygen

in water (approximately 10,000 times slower than in air) is not sufficient to replenish

the oxygen consumed by the microbial decomposers Therefore, decomposition of

the detritus is not complete, resulting in accumulation and burial of partially

decom-posed organic matter In this manner, some of the contaminants originally taken up

by plants can be trapped and stored as peat Peat may accumulate to great depths

in wetlands and can provide long-term storage for contaminants However, peat is

also susceptible to decomposition if the wetland is drained or soils otherwise

exposed, in which case the contaminants incorporated in the peat may be released

and either recycled or flushed from the wetland

Although microorganisms may provide a measurable amount of contaminant

uptake and storage, their metabolic processes play a much more significant role in

the removal of organic compounds Microbial decomposers, primarily soil bacteria

associated with the native organic matter in wetlands, use organic carbon (C) as a

source of energy converting it to carbon dioxide (CO2) or methane (CH4) gases This

affords a biological mechanism for removal of a wide variety of organic C

com-pounds including those found in municipal wastewater, food processing wastewater,

pesticides, and petroleum products The efficiency and rate of organic C degradation

by microorganisms are highly variable among different types of organic compounds

Microbial metabolism also affords removal of inorganic nitrogen, that is, nitrate

and ammonium, in wetland soils Certain bacteria (e.g., Pseudomonas spp.)

meta-bolically transform nitrate into nitrogen gas (N2), a process known as denitrification

The N2 is subsequently lost to the atmosphere, thus denitrification represents a means

for permanent removal, rather than storage, of nitrogen by the wetland Removal of

ammonium in wetlands can occur as a result of the sequential processes of

nitrifi-cation and denitrifinitrifi-cation Nitrifinitrifi-cation, the microbial (Nitrosomonas and

Nitro-bacter) transformation of ammonium to nitrate, takes place in aerobic regions of the

soil and surface water The newly formed nitrate can then undergo denitrification

when it diffuses into the deeper, anaerobic regions of the soil The coupled processes

of nitrification and denitrification are universally important in the cycling and

bio-availability of nitrogen in wetland and upland soils

Chemical Removal Processes

In addition to physical and biological processes, a wide range of chemical

processes are involved in the removal of contaminants in wetlands The most

important chemical removal process in wetland soils is sorption, which results in

short-term retention or long-term immobilization of several classes of contaminants

Sorption is a broadly defined term for the transfer of ions (or molecules with positive

or negative charges) from the solution phase (water) to the solid phase (soil)

Sorption actually describes a group of processes that include adsorption and

pre-cipitation reactions

Adsorption refers to the attachment of ions to soil particles, either by cation

exchange or chemisorption Cation exchange involves the physical attachment of

cations, or positively charged ions, to the surfaces of clay and organic matter particles

in the soil Cations are bonded to the soil by electrostatic attraction, a much weaker

force than chemical bonding; therefore, the cations are not permanently immobilized.

Many contaminants in wastewater and runoff exist as cations, including ammonium

(NH4, a plant nutrient) and trace metals such as copper (Cu2+) The capacity of soils

for retention of cations, expressed as cation exchange capacity (CEC), generally

increases with increasing clay and organic matter content Chemisorption represents

a stronger and more permanent form of bonding than cation exchange A number

of metals and organic compounds can be immobilized in the soil via chemisorption

with clays, iron (Fe), aluminum (Al) oxides, and organic matter Phosphate can also

bind with clays and Fe and Al oxides through chemisorption

Phosphate can also precipitate with iron and aluminum oxides to form new

mineral compounds (Fe- and Al-phosphates) that are potentially very stable in the

soil and afford long-term storage of phosphorus In the Everglades and other wetlands

that contain high concentrations of calcium (Ca), phosphate can precipitate to form

Ca-phosphate minerals which are stable over a long period of time Another

impor-tant precipitation reaction that occurs in wetland soils is the formation of metal

sulfides which are highly insoluble and are, therefore, an effective means for

immo-bilizing many toxic metals in wetlands

Volatilization, which involves diffusion of a dissolved compound from the water

into the atmosphere, is another potential means of contaminant removal in wetlands

Ammonia (NH3) volatilization can result in significant removal of nitrogen if the

pH of the water is high (greater than about 8.5) However, at neutral or low pH,

ammonia nitrogen exists almost exclusively in the ionized form (ammonium, NH4)

which is not volatile Many types of organic compounds are volatile and are readily

lost to the atmosphere from wetlands and other surface waters Although

volatiliza-tion can effectively remove certain contaminants from the water, it may prove to be

undesirable in some instances, due to the potential for polluting the air with the

same contaminants

PLANNING AND DESIGN

Constructed and natural wetlands have been used extensively to treat many types

of wastewaters and other contaminated waters such as urban and agricultural runoff

High levels of removal can be achieved for a number of contaminants, including

suspended solids, nutrients, metals, and organic compounds, in treatment wetlands

However, there are inherent limitations to the effectiveness of wetlands for

waste-water treatment In some cases, it may not be possible to achieve the desired level

of concentration reduction due to natural background levels Also, there is a relatively

high degree of time-dependent variability in treatment efficiency associated with

wetlands, especially when compared with conventional treatment technologies

(Kadlec, 1997) Because the contaminant removal interactions among vegetation,

soils, and hydrologic wetland components are complex, contaminant removal

effi-ciency varies widely among the types of treatment wetlands Moreover, the actual

pollutant loading that the treatment wetland can accommodate also varies

Treatment performance criteria for contaminant removal in wetlands may be

based on the contaminant concentration in the wetland outflow or on the total or

percent mass removal of the contaminant As a case in point, the efficiency of

nutrient removal decreases significantly as inflow concentration approaches the

natural background concentration of the nutrient in the wetland, even though the

outflow concentration may be well within the desired range Conversely, nutrient

removal efficiency, in terms of mass removal, may increase substantially as the

loading rate is increased to moderate levels, yet the outflow concentration may

exceed the desired level It is important that the selected criteria accurately reflect

the actual performance of the wetland relative to the objectives and intended uses

of the wetland treatment system The actual performance of treatment wetlands is

generally dependent on a multitude of factors, including inflow concentration, mass

loading rates, wetland design, and climate

TREATMENT WETLANDS AS A UNIT PROCESS

For proper design and operation of treatment wetlands, it is important that they

be considered part of an overall water treatment train Because of this, the

contam-inant removal effectiveness of the treatment wetland will be influenced by the

performance of the upstream unit processes Examples of upstream unit processes

for wetlands include sedimentation ponds or deep forebays for wetlands treating

urban runoff and primary clarifiers and/or secondary treatment processes (e.g.,

acti-vated sludge) for wetland treatment of domestic wastewaters Similarly, treatment

wetlands are not always the final unit process in a treatment train Wetlands can be

followed by filtration or disinfection processes

The treatment train concept is critical to consider for wetland treatment for two

reasons First, an overall assessment of the strengths and weaknesses of the unit

processes allows for a financially as well as technically optimized system Second,

the performance of the treatment wetland will be dictated in part by the quality (e.g.,

average, variability) of the water discharged by the upstream unit processes For

example, the treatment train might include a conventional, advanced secondary

domestic wastewater treatment plant (WWTP) to nitrify ammonium, a treatment

wetland designed to provide further N removal through denitrification, and

disin-fection by chlorine If during the lifetime of the treatment system the WWTP

becomes overloaded and fails to nitrify the wastewater ammonium, then the

wet-land’s effluent quality will dramatically decline In turn, ammonium discharged from

the wetland would reduce the effectiveness (or increase the cost) of the subsequent

disinfection unit process

Regulatory Issues

Federal, state, and local regulations must be carefully reviewed before using a

constructed or natural wetland for water treatment in the United States Almost all

point discharges into wetlands, whether municipal, agricultural, or industrial

waste-waters, will require a National Pollutant Discharge Elimination System (NPDES)

permit Federal regulations on the use of constructed wetlands for wastewater